Conductance switching is the basis of many potential molecular electronic devices, and has been the focus of numerous research efforts in recent years. If a single molecule or an assembly of molecules can be switched between a high conductance state and a low conductance state by an electrical or optical stimulus, molecular scale memory and logic elements become possible, and molecular electronic components may be integrated with conventional microelectronics or be assembled into true molecular circuits. A range of molecular structures that exhibit conductance switching have been reported, including self-assembled monolayers (SAMs) of phenylethylnyl oligomers, rotaxanes oriented between two conductors by Langmuir-Blodgett assembly and nitroazobenzene thin films bonded to carbon. The phenomenon of negative differential resistance is notable as an example of a small collection of molecules changing from a high to a low resistance state in an applied electric field, then back to the high resistance state as the field was increased further. In several cases the states of the molecule are persistent and controllable, and may repeatedly cycle between “ON” (low resistance) and “OFF” (high resistance) states.
Although several examples of conductance switching have been extensively investigated, the switching mechanism in most cases is unknown, and the subject of some controversy. Negative differential resistance (NDR) was originally attributed to redox reactions involving a nitro and/or amino group in the monolayer molecule, but switching was subsequently observed without such groups present. NDR was also attributed to shifts in molecular orbital energies in response to an applied electric field, resulting in resonant tunneling at certain applied field magnitudes. Switching by isolated phenylethylnyl molecules in mostly aliphatic SAMs was observed to be stochastic by scanning tunneling microscopy (STM), and was attributed to confirmation changes, possibly involving interactions with the monolayer surrounding the active molecules. Of course, any mechanism established to explain conductance switching also bears directly on the broader issue of which factors control electronic conductivity in organic molecules, a topic of wide interest in the areas of conducting polymers, opto-electronic materials and energy conversion, in addition to molecular electronics.
We report here a completely new approach to fabricating molecular junctions that exhibit conductance switching, and which provide critical insights into the switching mechanism. The present application hereby incorporates, in its entirety, by reference application Ser. No. 09/755,437, filed Jan. 5, 2001, entitled “Chemical Monolayer and Micro-Electronic Junctions and Devices Containing Same”.
“Conductance switching” refers to a change in conductivity of a molecular junction. The molecule acts as an electronic conductor that has at least two distinct states, one of low resistance and one of high resistance. In response to some stimulus, the molecule can be made to change conductance states. Switching has been reported for several molecular junctions, but never with carbon substrates. The mechanism is controversial and the chemical requirements for a successful molecular switch are unknown.
The large contemporary interest in molecular electronics has its origin in several diverse areas, including conducting polymers, long-range electron transfer, and molecular transistors. Combined with nanofabrication techniques, molecular electronics concepts hold great promise for creating new molecular devices for information storage and processing, as well as chemical and optical sensing. Two aspects of molecular electronics are particularly relevant to the current proposal: sublithographic fabrication and the exploitation of molecular properties in electronic devices. Much of the huge research effort in nanotechnology expended to date was directed toward nanoscale or single molecule devices, driven by the promise of much higher data density and processing speed. Such devices are below conventional lithographic size scales and would require novel methods of assembly. Single molecule or nanoscale electronic devices hold exceptional promise for technical and economic value, but also present formidable hurdles for practical utility, and massively parallel fabrication.
The approach described herein is directed toward the second major area of molecular electronics: the exploitation of molecular properties for electronic, chemical, and optical functions. A molecular junction of the present invention is nanoscale” (5-50 Å) in one dimension, but may be incorporated into a lithographic structure which is micron-scale in the other two dimensions. The use of a carbon substrate and conjugated, covalent bonding to a monolayer is a completely new approach to forming molecular junctions, and may have some profound consequences described below. Given the huge range of structures that might be incorporated into a molecular junction, a correspondingly large range of electronic properties is expected. As examples, molecules have a variety of energy levels analogous to semiconductor band gaps, and it should be possible to make a junction that can sense or produce photons, store charge, and interact with nearby molecules in a sensor.
In order to demonstrate the novelty of the present invention, a brief overview of the general area of molecular electronics is necessary. The general area has been quite active since approximately 1990, with a primary target being demonstration of current flow through molecules. Metal/thiol and Langmuir-Blodgett structures have demonstrated very interesting properties about molecules as components in electronic circuits, including Coulomb staircases, Schottky barriers, rectification, charge storage and staircase current/voltage curves. Molecular memory devices and multistate logic elements made from molecules assembled by the Langmuir-Blodgett technique have received wide recognition in both the scientific and popular press. Molecular junctions involving alkane chains between two mercury drops or between mercury and silver have also been examined, and compared to the behavior expected from tunneling. Although the gold/thiol and Langmuir-Blodgett structures used to fabricate these molecular devices have been widely studied, they have yet to yield a commercial electronic product.
A different approach related to the present invention is based on conducting organic polymers and semiconductors, which may be fabricated into analogs of traditional inorganic semiconductor devices such as transistors, light emitting diodes, and photosensors. The recent Nobel Prize in Chemistry, shared by Alan MacDiarmid, Alna Heeger, and Hideki Shirakawa recognized the promise of conducting polymers for electronic devices, and this technology is the basis of a start-up company (Uniax) recently purchased by DuPont. Although there is great promise for conducting polymers in electronic devices, light emitting diodes, and chemical sensors, the current approach differs fundamentally. The organic layer of the present invention is monomeric and oriented, unlike conducting polymers, and covalent bonding of contact and monolayer results in strong electronic coupling between contacts and molecules.
The field of electrochemistry is based on electron transfer between a conductor and a molecule, sometimes through a monolayer film, and some electrochemical principles are directly applicable to molecular electronics. The conceptual distinction between electrochemistry and molecular electronics is important, however, even though both may involve transduction between chemical and electronic events. Electrochemistry transforms a chemical event to electronic current flow by a process (such as a redox reaction) which has an activation barrier manifested as reorganization energy and other work terms. In contrast, the behavior of a molecule in an electronic circuit need not involve a chemical activation barrier or ionic motion; it may be viewed as totally electronic. Electrochemical signal transduction combines “chemical” and “electronic” phenomena, while signal transduction via molecular electronics may be entirely “electronic”. The wide range of available molecular structures should impart equally wide variability to an electronic circuit, whether the circuit is totally “molecular” or a hybrid of molecular and conventional electronics. The potentially powerful aspects of molecular electronics to be exploited herein are the control and/or modulation of electronic circuit behavior by chemical events, and exploitation of molecular variety for advanced sensing and information processing.
This brief review indicates the promise of molecular electronics for various applications, including high data density storage, small and multifunctional processors, and possibly self-assembling complex devices. A few general observations about existing techniques are relevant to the approach proposed here. First, the Au/thiol SAM is widely studied and relatively simple to prepare, but is prone to pinholes generally limiting SAM devices to small areas (˜30×20 nm). In fact, the SAM system must have a finite density of pinholes at equilibrium in order to exhibit self-assembly. Second, the relatively weak interactions responsible for orienting Au/thiol and Langmuir-Blodgett (LB) structures are thermodynamically prone to disordering, since the bond energies are 40 kcal/mole or less (about 1.6 eV). Third, both Au/thiol and LB structures result in an energy barrier between the conductor and the molecule. The polar, partly ionic Au—S bond represents an ˜2 eV barrier for SAMs, while LB structures are often positioned on metal oxide films having high resistance (>1 MΩ). No structure yet reported has an ohmic contact between molecule and conductor, thus hindering the inclusion of molecular properties into an electronic circuit. Fourth, molecular devices with sizes below the current lithographic limit (˜100 nm) are difficult to fabricate in a massively parallel device, at least with any currently available methods. So the problem of parallel fabrication must be surpassed for the case of sub-lithographic devices, in order to produce cost-effective components or integrated circuits. In summary, before a practical device that harnesses the power of directly combining molecular properties with electronic conduction can be realized, several milestones must be achieved:                Low resistance, ohmic contact between conducting contacts and molecule.        Incorporation of molecular properties (tunable band gap, chemical and optical sensitivity) into electronic circuit.        Robust, long-lived performance with indefinite shelf life.        Massively parallel fabrication, at or below the lithographic limit.        Compatibility with conventional microelectronics in a hybrid circuit.        